Plasma treatment is frequently used to modify the surface properties of substrates used in a diversity of applications including, but not limited to, integrated circuits, electronic packages, and printed circuit boards. In particular, plasma treatment may be used in electronics packaging, for example, to etch resin, to remove drill smear, to increase surface activation and/or surface cleanliness for eliminating delamination and bond failures, to improve wire bond strength, to ensure void free underfilling of chips attached to printed circuit boards, to removing oxides from surfaces, to enhance die attach, and to improve adhesion for chip encapsulation.
In a conventional plasma processing system, multiple substrates are placed inside a vacuum chamber, the vacuum chamber is evacuated and filled with a partial pressure of a source gas, a plasma consisting of a partially ionized source gas is generated inside the vacuum chamber, and a surface of each substrate is exposed to the plasma species. The outermost surface layer(s) of atoms are removed from each substrate by physical sputtering, chemically-assisted sputtering, and chemical reactions promoted by the plasma. The physical or chemical action may be used to condition the surface to improve properties such as adhesion, to selectively remove an extraneous surface layer, or to clean undesired contaminants from the substrate's surface.
In multiple electrode plasma processing systems, both sides of multiple panels of material are simultaneously plasma treated in a batch process. A substrate holder holds each panel with a vertical orientation between a pair of planar vertical electrodes, which are energized with a suitable atmosphere present in the treatment chamber of the treatment system to generate the plasma. The environment between each planar vertical electrode and the adjacent surface of the panel supplies a local process chamber in which the partially ionized source gas is present.
Plasma processing may be used during the manufacture of printed circuit boards to process the panels during various stages of fabrication. In one application, plasma is used to remove drill smear having the form of residual polymer resin that is spread along the walls of throughholes and vias by a mechanical drilling process. If left unmitigated, the drill smear may interfere with the plating process, such as electroless copper plating, used to form the metallization that establishes the electrical connections of the printed circuit board. Plasma may also be used to activate the surfaces of polymers used in the printed circuit boards to increase the wettability and lamination of plated metallization applied to the walls of throughholes and vias. Plasma may also be employed to remove photoresist residue when patterning fine pitch metallization lines on printed circuit boards and to prepare the inner layer of flex materials, such as polyimides, for metallization. Plasma may also be used to remove carbon residue resulting when the printed circuit board is laser drilled to form blind vias.
The panels that are plasma processed using multiple electrode plasma processing systems may be quite large. For example, the panels may have a rectangular perimeter that is characterized by a width of about 26 inches and a length of about 32 inches. Process uniformity across the entire surface area of each board achieved by conventional processing systems, while adequate for their intended purpose, may be insufficient as technological advances increase the density of the interconnects on the printed circuit board. Another challenge is to uniformly treat the panel in each of the different process chambers
While conventional plasma processing systems have been adequate for their intended purpose, there is a need for a plasma processing system that is capable of improving the uniformity of the processing over the entire surface area of each printed circuit board and among multiple printed circuit boards processed in any single processed lot of boards.